I'm really excited to see someone working on this problem. Great idea!

As for further graphing options, don't forget Zimbra provides some basic graphs internally via their management and monitoring interfaces. On my installation these include message volume and counter information as well as antispam and antivirus details.

These graphs require Flash which mean they're not suitable for my iPad, but Cacti works perfectly.

I'd also like to see active connections graphed, broken down by protocol (POP3, IMAP, SSL versions of both).

You might want to look at other MTA systems for inspiration too (we use an IronPort and there's some great templates for that on the Cacti forum).

Thanks again, and I'll keep an eye on this project. Are you tweeting updates on Twitter?